In the manufacture of hydraulic machine runners for turbines, it is known to bolt the runner blades of the turbine onto the runner hub of the turbine with fitted studs and nuts. Typically the hub is hollow and has a non-planar outer surface that is conical, cylindrical or spherical in shape. The blade is provided with a circular shaped flange which is bolted to the outer surface of the runner hub by a series of studs and nuts arranged in a single circle that pass through the runner hub and into the flange of the runner blade. The bolted connection allows for an easy adjustment of the blade angle to meet homology requirements during the assembly of the hydraulic machine and reduces runner size for transportation as the blades are not welded in place and can be removed.
In some hydraulic turbine assemblies, space limitations within the runner hub prevent a sufficient number of bolting connections, arranged in the single circular pattern on a circular flange, to pass through the hub and into the circular flange. As a result it may not be possible to ensure proper fatigue life of the bolting by flange pre-loading. Further, when the space limitations result in the flange having a smaller diameter than desired, stress levels in the flange can rise over acceptable limits. In these instances the runners are usually assembled without the use of bolting and by welding the runner blades to the runner hub or with the use of segmented casting that is united with shrink rings or welding.
Clearly, any improvements to the connection between the runner hub and the runner blade flange which reduce stress levels would be desirable.